The invention relates to a cutting insert for a high-speed cutting operation, and more particularly, to a high-speed milling cutter that includes a taper pin that forces the cutting insert against the radial seating wall of the insert pocket to minimize or eliminate movement of the cutting insert and the resulting bending moment and shear forces on the insert mounting screw.
Milling cutters for performing machining operations on metallic work pieces are well known in the prior art. Such cutters typically comprise a cylindrical or disc-shaped body which is detachably connectable to a rotating drive shaft. Cutting inserts are mounted around the outer periphery of the cutter body for producing a series of metal-shaving cuts on a work piece. In operation, such milling cutters are typically rotated at speeds of several thousand rpm while a work piece is engaged with the inserts mounted on the cutter body.
Recently, there has been an increased demand for milling cutters capable of operating at rotational speeds far in excess of several thousand rpm. The advantages associated with such high-speed milling include a faster cutting action which results in a higher metal removal rate on the work piece, a reduction in the cutting forces applied to the cutting inserts by the work piece, and a smoother final cut. Such reduced cutting forces protract the operating life of the inserts, not only reducing the costs associated with insert replacement, but also the amount of downtime necessary to reorient the cutting edges of indexable inserts. The cost and time of fixturing is also reduced because higher cutting forces require more elaborate and more rigid fixturing to achieve desired accuracy.
As a result of these advantages, a high-speed milling cutter not only lowers machining costs while increasing productivity, but also enhances the quality of the final machined work piece since the cutting action is smoother, and leaves a better finish. It will be appreciated that the substantial increase in rotational speed necessary to obtain all the aforementioned advantages also results in a substantial increase in the centrifugal forces generated in the body of the cutter. Generally speaking, the centrifugal force Fc is dependent upon the mass (m) of the cutter body supporting the cutting insert, the length of the radius (r) of the cutter body, and the square of the angular velocity (Ω) of the body. The relationship between these parameters may be expressed in the equation Fc=(mΩ2)(r). The fact that the centrifugal force (and hence tensile stress) on the cutter body increases with the square of the angular velocity has, up to now, posed a substantial obstacle in the development of a milling cutter capable of operating at speeds higher than several thousand rpm. A milling cutter rotating at 10,000 rpm would have 25 times more centrifugally induced tensile stress along its periphery than when it was operated at 2,000 rpm. If the same cutter is spun at 20,000 rpm, it would have over 100 times more centrifugally induced tensile stress.
In addition, the substantial increase in rotational speed necessary to obtain all the aforementioned advantages also results in a substantial increase in the centrifugal forces generated on the inserts of the cutter. Specifically, the centrifugal forces tend to cause the inserts to become unseated from the insert pocket during high-speed milling operations.
Currently, a couple of different designs reduce, but not eliminate, the bending moment encountered by the insert mounting screw. One design incorporates a rail on the insert and a corresponding groove in the cutter body. In this “Rail Design”, the insert is designed to seat on the pocket floor, and the axial and radial walls of the cutting insert transfer the cutting forces to the cutter body. Because the cutting insert is seated in this manner, the rail and groove must be designed with clearances between faces. These clearances are necessary to ensure that the cutting insert seats only on the pocket floor and the axial and radial walls. The rail clearance is driven by achievable manufacturing tolerances on the insert and cutter body. Because of the clearance between the rail and groove, and the limit on movement of the cutting insert, the insert mounting screw may still experience bending moments at high rpm, but at a reduced amount. Also, a condition that could promote cutter failure at high rpm is the groove in the cutter body reducing pocket strength by reducing the cross sectional area of the material in the pocket floor.
Another design incorporates a raised boss in the pocket floor that provides additional support around the threaded section of the insert mounting screw to reduce the bending moments created at high rpm's. In this “Raised Boss Design,” the cutting insert is designed with a counter bore to provide clearance for the raised boss.
Although both the “Rail Design” and the “Raised Boss Design” reduce the bending moments on the insert mounting screw, there is still a need for a high-speed milling cutter capable of operating at high speeds, for example, about 20,000 rpm that securely and positively retains the cutting inserts within the insert pockets of the cutter body. Ideally, such a high-speed milling cutter and cutting inserts should be relatively inexpensive to manufacture, and should utilize inexpensive, readily replaceable cutting inserts so as to minimize both the cost of fabrication and operation of the device.